Compressors and turbines of gas turbine engines such as those used in aero engines typically include a plurality of rotor and stator vane assemblies. The rotor assemblies are designed to impart work into gases passing through the compressor and extract work from the gases passing through the turbine. The stator vane assemblies help to direct the working gases entering or exiting the rotor assemblies and thereby increase efficiency of the engine.
Each rotor assembly includes a disc and a plurality of blades attached to the disc so as to extend radially outwardly therefrom. Conventionally, the blades are attached to the disc by mechanical connections such as “fir tree” type connections where a fir tree shaped blade root is received within a complementary shaped recess in the disc. This means that the blade can be readily replaced in the event that it is damaged.
Recent developments have resulted in integrally bladed rotor assemblies or “blisks” in which the blades are formed integrally with the disc. These have advantages of reduced weight as compared to a standard rotor assemblies and improved aerodynamic efficiency.
For example, blisks can be formed by joining the blades onto the rim of the disc at respective blade stubs by linear friction welding (LFW). This is a process whereby one part is held stationary while the other part is oscillated against it under a forge load, the heat generated and the applied loads result in a weld, as upset material exudes as flash from the edges of the joint. In the repair of blisks, the replacement blade is generally oscillated relative to the disc, while the forge load is applied in the radial direction on the blade towards the disc or on the disc towards the blade. The blade is thereby joined to the disc.
In view of the nature of their applications, blisks are susceptible to damage that in some circumstances may require the replacement of a blade. In order to replace the blade, it can be removed by machining it off to leave a repair stub, and welding a new blade onto the repair stub by LFW.
LFW results in the consumption of material (which leaves the edges of the weld as flash) and in the recirculation of contaminants back towards the weld during the process. Consequently, a large surface area is required to allow a good quality weld to be formed. This results in a necessity to machine off significant amounts of material around the weld. US 2005/0274010 (hereby incorporated by reference) proposes a collar which provides support for the stub during the friction welding process.
The majority of defects within the LFW process occur at the leading and trailing edge of the blade. Further it is difficult to perform non-destructive evaluation of blisk LFW welds. Accordingly when forming an original blisk it is usual to provide a “clean-up region” which is machined off and does not form part of the final blade. The shape of the original stub follows strict validated guidelines which ensures that the flow of material from the weld behaves similar to previous experience and provides a statistical confidence that no defect is present in the weld after machining to remove excess material.